APPLICANT'S ABSTRACT: Naltrexone treatment reduces drinking rates for alcoholics, especially for those who lapse. However, the mechanisms that may mediate naltrexone's effects on drinking are not well understood. Effects on urges may provide one mediating mechanism. Clinical trials suggest that naltrexone reduces urges to drink, and lab studies with alcohol showed that naltrexone reduced the probability of experiencing cue-elicited urges to drink. Effects on the reinforcing effects of alcohol after taking a drink may provide another set of mediating mechanisms. Lab studies with heavy drinkers found brief administration of naltrexone to decrease the pleasurable effects of drinking, increase latency to a second drink, and reduce drinking rates. Naltrexone may thus reduce additional drinking due to reductions in positive reinforcement. The overall primary objectives are to determine whether naltrexone: (1) reduces urges to drink and initial reactions to drinking among heavy drinkers in the natural environment, using ecological momentary assessments (EMA), and in laboratory assessments of urge in response to alcohol cues, and (2) reduces drinking rates in this population. Therefore, we propose to provide 3 weeks of naltrexone (50 mg/day) or placebo administration to heavy drinkers in a 2-group randomized placebo-controlled design. All will participate in a week of baseline recording, a week of placebo leadin, then 3 weeks randomized to medication condition with medication management sessions. All participants will use palmtop computers daily to record urges to drink, environmental circumstances in which urges occur, drinking behavior, and reactions to drinking. All will participate in a lab assessment of alcohol cue reactivity after one week on the medication. This study will be the first to investigate the effects of naltrexone with heavy drinkers in a fine-grained analysis in the natural environment. The results will add important information about some of the hypothesized mechanisms of naltrexone's effects and about the relationships among environmental circumstances, urges and drinking behavior. Since heavy drinkers pose a substantial risk to other people and cost to society, investigation of the effects of naltrexone on this population may ultimately lead to interventions for heavy drinkers who need to modify their drinking rates.